1. Field of the Invention
The present invention relates to improvements in an electric power steering device and a method of assembling the electric power steering device.
2. Description of the Related Art
In recent years, electric power steering devices have been used extensively as they can reduce the driver's muscular effort or force required to turn the steering handle and thus provide a comfortable steering touch to the driver. The electric power steering devices of the type concerned are constructed such that an auxiliary torque generated by an electric motor in proportion to the steering torque is transmitted via a mechanical clutch to the steering system. One example of such electric power steering devices is disclosed in Japanese Patent Laid-open Publication No. HEI 1-172058, entitled "Electric Power Steering Device".
The disclosed electric power steering device includes a steering torque detecting means for detecting a steering torque in a steering system, and an electric motor for generating an auxiliary torque according to a detection signal from the steering torque detecting means. The electric motor is connected to a sleeve, and the sleeve is connected by a clutch mechanism to a hexagonal portion of an output shaft. The output shaft is connected to steered wheels. Thus, the auxiliary torque from the electric motor is transmitted to the output shaft. The sleeve forms part of the clutch mechanism and is rotatably supported on an upper housing. The output shaft is rotatably supported on the upper housing and a lower housing. The output shaft also has a portion supported on the upper housing via the sleeve. The conventional electric power steering device having such support or bearing structure has a limited assembling accuracy.
The steering torque detecting means, in one form, includes a ring fitted around an input shaft, and a strain gauge attached to a cantilevered resilient metal strip. The resilient metal strip has a free end held in engagement with the ring. The ring is movable or displaceable in the axial direction of the input shaft in response to a relative torsional displacement or rotation between the input shaft and the output shaft. With this displacement of the ring, the resilient metal strip is resiliently deformed or flexed whereupon the strain gauge produces an electric signal corresponding to the relative torsional displacement between the input shaft and the output shaft.
In an alternative form, the steering torque detecting means includes two confronting annular flanges formed on the input shaft and the output shaft, respectively, and a brush slidably disposed between the annular flanges so as to form, jointly with the annular flanges, a slip ring. The thus formed slip ring generates an electric signal directly corresponding to a relative torsional displacement between the input shaft and the output shaft.
In the first-mentioned form of the steering torque detecting means, the resilient metal strip carrying thereon the strain gauge is cantilevered within a case. Since the case is attached to the upper housing, mounting of the steering torque detecting means should be achieved in the course of an assembling process which is performed to assemble a body of the electric power steering device in a housing composed of the upper housing and the lower housing. Such a mounting procedure is quite laborious and makes it difficult to replace the steering torque detecting means. In addition, since an adjustment of the neutral position of the steering torque detecting means is performed under the condition that the steering torque detecting means is held in the housing, it is very difficult to carry out the neutral position adjustment work with sufficient efficiency and accuracy.
The second-mentioned form of the steering torque detecting means also has a drawback that since two confronting annular flanges forming the slip ring are formed on the input shaft and the output shaft, respectively, replacement of the steering torque detecting means is practically impossible. The input shaft further has a large-diameter tubular portion forming one part of the clutch mechanism, and the output shaft further has a hexagonal portion forming another part of the clutch mechanism. With this arrangement, an adjustment of the neutral position of the steering torque detecting means and an adjustment of the neutral position of the clutch mechanism cannot be achieved separately. The neutral position adjustments are, therefore, rendered tedious and time-consuming and insufficient in accuracy.